This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for coalescing and affixing permanently a powder pattern to a support material in image configuration.
In the process of electrophotographic printing, a charged photoconductive member is exposed to a light image of an original document to be reproduced for recording thereon an electrostatic latent image. A development system, thereupon, moves a developer mix of carrier granules and toner particles, into contact with the photoconductive member. Toner particles are attracted electrostatically to the latent image forming a toner powder image thereon. The toner powder image is, then, transferred to the sheet of support material.
Multi-color printing repeats the foregoing process a plurality of cycles. Each development cycle deposits differently colored toner particles on the support material, in superimposed registration with the previously deposited toner particles. Hence, the support material will have transferred thereto various layers of toner particles forming a multi-layered powder image. The powdered layers have to coalesce and become transparent, i.e. each toner layer modulates the light rays passing therethrough, to form a copy having a single composite color. In this manner, the modulated light rays transmitted through the toner powder layers are reflected from the support material back through the toner powder to the eye of the observer. If the toner layers do not become transparent, the color reproduced will merely be that of the uppermost layer of toner particles. It is apparent that the fusing operation must coalesce the toner particles and form transparent layers thereof to appropriately modulate the light rays transmitted therethrough creating a copy having the composite color of the original document.
Generally, the toner particles include, primarily, fusible resins. When such toner particles are transferred to the support material, the powder image can be permanently affixed thereto by heating which partially dissolves the toner particles causing them to fuse into the support material. Conventional black and white electrophotographic printing machines utilize fusers which heat the toner particles sufficiently to fix them to the support material, while not charring or igniting the support material. Heretofore, there was little concern with coalescing multi-layers of toner particles. Thus, it is apparent that the requirement for coalescing multi-layers of toner particles adds but one more variable in an already complex system.
Preferably, it is desirable to raise the temperature of the support material so that it is close to the fusing temperature of the toner particles. In this way, the support material acts as a heat source rather than a heat sink during the fusing operation. Thus, a suitable fusing apparatus may include heating the support material as well as applying radiant heat thereto to permanently affix and coalesce the toner powder image to the support material.
Heating of the support material prior to exposing the toner powder image to radiant energy has been disclosed in U.S. Pat. No. 3,187,162 issued to Toku Hojo et al. in 1965. As disclosed therein, the support material advances over a pre-heat platen which raises the temperature thereof by conduction. The fuser disclosed in the Hojo patent also includes an incandescent filament lamp arranged adjacent to and over the path of travel of the support material to emit radiant energy which assists in fusing the toner powder image to the support material.
In high speed electrophotographic printing machines, the utilization of a stationary pre-heat platen may pose a problem. A boundary layer of air frequently is developed between the rapidly moving support material and the pre-heat platen. This layer of air serves to insulate the support material from the pre-heat platen and introduces a variable in the actual temperature of the support material. The more rapidly the support material is moved over the platen, the greater the uncertainty in heating the support material to a predetermined temperature. This is particularly significant when the support material is cut sheet paper, such as is used in conventional electrophotographic printing machines, rather than a web of paper passing over the pre-heat platen. The rapid movement of a relatively non-rigid sheet forces air between the heated platen and the sheet. Hence, the more rapidly the cut sheet of paper moves over the heated platen, the greater will be the boundary layer created thereby. In addition thereto, the radiant heater and the pre-heat platen must operate in conjunction with one another to coalesce and affix the multi-layer toner powder image to the support material. This should be achieved without charring or igniting the support material. Precise temperature control of the support material is not readily attainable when a sheet of cut paper passes over a stationary platen forming a layer of air therebetween.
Accordingly, it is a primary object of the present invention to improve the fusing of single or multi-layered toner powder images to create a composite colored reproduction corresponding to the original colored document.